The organic EL element comprises a pair of electrodes consisting of an anode and a cathode, and a light-emitting layer provided between these electrodes. An organic EL element in which one layer of light-emitting layer is provided is called a “single photon-type organic EL element” and an organic EL element in which a plurality of light-emitting layers are provided is called a “multi-photon-type organic EL element”.
When a voltage is applied to the organic EL element, a hole is injected from the anode and an electron is injected from the cathode. Then, the injected hole and the injected electron are coupled with each other in the light-emitting layer to generate light emission.
In the multi-photon-type organic EL element, for the purpose of lengthening the life of the element or enhancing the brightness of the element, a plurality of light-emitting layers are provided and usually, between the light-emitting layers, a charge generation layer is provided. As a material for the charge generation layer, an alkali metal, an alkaline earth metal, an inorganic semiconductor material, a charge transporting organic material, and the like are used mainly from the viewpoint of electric characteristics. The charge generation layer is made up, for example, by layering an electron generation layer made of an inorganic semiconductor material having electron injection characteristic and a hole generation layer having hole injection characteristic. The electron generation layer or the hole generation layer is formed by co-depositing the above-described materials or by depositing the above-described material individually (see Patent Document 1).